This invention relates to communication equipment. More particularly, it is concerned with amplitude modulated radio transmitters having a control loop for regulating the carrier output power.
It is common practice for amplitude modulated radio transmitters to be designated to operate at a normal carrier output level that is of the order of one-fourth the power required during peaks of modulation. The maximum peak power available typically is in excess of four times the normal carrier level in order to allow for proper operation under adverse conditions. Since the extremely high levels of modulation are present for only very short periods of time, it is not economical or practical to design transmitters for continuous operation at high peak power levels.
In amplitude modulated radio transmitters employing low level modulation a DC control loop is frequently employed to maintain the carrier output constant. The output power from the final power amplifier of the transmitter is monitored by an RF coupler and detector arrangement, including a filter to remove any modulation which may be present, in order to produce a DC voltage proportional to the carrier output power. This DC voltage is compared with a reference voltage which may be variable in order to permit the carrier output level to be set. The resulting DC control voltage is applied to a low power RF amplifier stage to control its output and thereby maintain the carrier output at the desired level. The audio signal to be transmitted is combined with the DC control voltage and applied to the low power amplifier stage to amplitude modulate the carrier signal.
In transmitting apparatus of the foregoing type a failure or discontinuity in an element included in the carrier control loop may cause the DC control voltage to rise, thereby causing the transmitter to attempt to produce maximum carrier output power. Since the transmitter is not designed for continuous operation at the maximum level, other elements will be overdriven and one or more elements may be destroyed. That is, the control loop when disrupted by a relatively minor component failure may cause other components to be overdriven and burned out thereby causing complete failure and additional damage to the transmitter.